1. Field of the Invention
The present invention relates to an epoxy composition for encapsulating a semiconductor element and a semiconductor device, and more specifically relates to an epoxy composition for encapsulating a semiconductor element and a semiconductor device formed using the epoxy composition.
2. Description of the Related Art
Until now, a semiconductor element incorporated in (provided in) a semiconductor device is encapsulated using an epoxy resin composition which is prepared by mixing a curing agent such as phenol resin and an inorganic filler such as fused silica or crystallized silica with epoxy resin having excellent heat resistance and moisture resistance.
Recently, in accordance with high level integration of an integrated circuit, a size of a semiconductor device becomes larger. In addition, a surface mounting type semiconductor device such as TSOP, TQFP or BGA are widely used. Such a surface mounting type semiconductor device is mounted on a substrate through a reflow soldering, and thus the semiconductor device is required to have higher heat stress resistance for the reflow soldering than that of a conventional semiconductor device due to its increased size.
In such a surface mounting type semiconductor device, due to the heat stress which would occur during the reflow soldering, there is a problem in that a crack is likely to be generated in the semiconductor device (a cured product of the epoxy resin composition), or delamination is likely to occur at a boundary surface between the semiconductor element and the cured product or a boundary surface between a structural element other than the semiconductor element and the cured product. Therefore, it is strongly demanded that an epoxy resin composition having excellent heat resistance is developed.
Further, in order to cope with the recent environmental problems, as for a solder used in a reflow soldering, one having a low lead content is preferably used. However, use of such a solder requires a higher temperature in a reflow soldering process. As a result, it is required that the epoxy resin composition used in such a surface mounting type semiconductor device has more excellent reflow soldering resistance which includes low stress property and low hygroscopicity in addition to high heat resistance.
For these reasons, as an epoxy resin composition to be used for the surface mounting type semiconductor device, recently, epoxy resin having excellent low stress property and excellent low hygroscopicity which are lower than those of conventionally used orthocresol novolak type epoxy resin and phenol novolak type resin is used.
However, in the case where such an epoxy resin is used, due to the chemical structure thereof, a glass-transition temperature of a cured product of the epoxy resin composition containing the epoxy resin is lower than that of an epoxy resin composition containing orthocresol novolak epoxy type resin and phenol novolak type resin.
In the case where the glass-transition temperature is low, since moisture content is relatively easily penetrated into the cured product under a humid condition, ionic impurities such as Cl− contained in the epoxy resin composition are easily movable in a semiconductor device formed using it. As a result, corrosion of the integrated circuit included in the semiconductor device is likely to occur due to contact with the ionic impurities. This means that the semiconductor device formed using such an epoxy resin composition cannot have sufficient moisture resistance that is required to maintain a function of the semiconductor device under the humid condition.
In order to trap the ionic impurities which cause corrosion of the integrated circuit, it is proposed that an ion scavenger containing Bi-based inorganic compound is mixed with the epoxy resin composition (JP-A 11-240937, pages 2 to 11), a magnesium oxide-based or aluminum oxide-based ion scavenger is mixed with the epoxy resin composition (JP-A 60-42418, pages 2 to 4), or a zirconium-based ion scavenger is mixed with the epoxy resin composition (JP-A 2002-371194, pages 2 to 6).
In these approaches, each epoxy resin composition could have slightly improved moisture resistance, but a degree of the improvement of the moisture resistance is not necessarily to be a satisfactory level.